There are a number of known indicators of cardiac output, with an example being the contractility of the right ventricle. The contractility describes the force and the speed of a myocardial contraction. The contractility is controlled by three mechanisms: direct control by the autonomous nervous system (ANS); the so-called Frank-Starling mechanism; and the so-called Bowditch effect (force-frequency coupling). The main mechanism, control of the circulatory regulation by the autonomous nervous system, increases the contractility and the heart rate when elevated metabolic demand exists, for example, in the event of physical or mental exertion, in order to ensure a suitable blood supply.
In patients having chronic heart failure (HF), the myocardial contractility decreases to a low level and the interventricular synchronization is worsened. This is accompanied by a low ejection fraction (EF), and by a lesser quality of life and a high mortality. HF occurs frequently in the population. HF patients are treated using various medications which influence the inotropic state in order to stabilize the heart rate, such as beta blockers, but positive inotropic medications, such as glycosides, are also used in order to elevate the contractility. More recently, HF patients have been treated using resynchronization therapy devices, such as three-chamber cardiac pacemakers or defibrillators. The goal of such cardiac resynchronization therapy (CRT) is to synchronize the two ventricles of a heart through biventricular stimulation, in order to improve the time behavior of the ventricles and thus the cardiac output.
The contractility is therefore an important variable to observe, in particular for HF patients. Such observation is important in order to observe the status of the patient and the alleviation or progress of their illness; to establish and observe the resynchronization therapy of the heart (cardiac resynchronization therapy, CRT); and/or to observe the status of treatment via medication. Information about the contractility can additionally be used for the purpose of optimizing cardiac pacemaker therapy or a therapy by an implantable cardioverter/defibrillator (ICD).
Although the contractility is a variable of great importance, it is difficult to measure in clinical practice. Determining the contractility on the basis of a maximum ventricular pressure gradient (dP/dt)max in the right or left ventricle is known. A left-ventricular ejection fraction (EF) can additionally be determined using echocardiography. Examination of the right ventricle using echocardiography is difficult for anatomical reasons, although information about the right ventricle is very important for a complete examination. Both procedures—the pressure measurement and the echocardiography—are time-consuming and costly. The ventricular pressure measurement requires an invasive procedure which situates a pressure catheter in one or both ventricles, and can only be performed during an electrophysiological study or during the implantation of a cardiac pacemaker or cardioverter/defibrillator
Implants which allow the contractility of a heart to be ascertained are described, for example, in U.S. Pat. Nos. 4,674,518; 5,417,717; and 7,519,422. A measurement of the change of the ventricle volume is performed therein using the pressure gradient dP/dt and using impedance plethysmography.